Typical corrosion-resistant martensitic steels used at high temperatures contain between 9 and 12 chromium, and 0.08 and 0.25 carbon (wt. %). These steels usually contain several additional carbide forming elements such as molybdenum, tungsten, vanadium and, in some cases, niobium. Additional elements such as silicon, nickel and manganese are also typically added to these steels to deoxidize, reduce delta ferrite formation, and getter the sulfur, respectively. The conventional heat treatment for these steels involves austenitizing in the range .about.1000.degree. C. to .about.1100.degree. C., air cooling to room temperature (which usually transforms most of the austenite to martensite or bainite) and tempering between .about.650.degree. C. and .about.750.degree. C. The tempered microstructure usually consists of relatively large, chromium-rich carbides which have nucleated on martensite lath boundaries, prior austenite grain boundaries and other crystalline defects in the ferrite matrix. The precipitate distribution in the tempered martensite is primarily responsible for the rather modest creep strength (to 600.degree. C.) of conventional 9-12 Cr steels. But at temperatures greater than 600.degree. C, these steels are not generally used due to their inferior creep properties. The reason for their inadequate high temperature strength is due to the relatively rapid coarsening kinetics of the chromium carbides. As the precipitates coarsen, the average interparticle spacing increases, which allows dislocations to glide more easily between particles.
A variety of ferritic steels having high chromium content have been proposed. Many of these steels are said to be creep resistant. Creep resistance is usually measured by applying a stress to the steel while the steel is at an elevated temperature, typically 600.degree.-700.degree. C. Then one measures either the creep strain over time (the steady-state creep rate) or the time which passes until the steel ruptures. The rupture time for most steels can be found in the literature or calculated. Under conditions of 200 MPA and 650.degree. C., many 9-12 Cr martensitic steels rupture within about 100 hours; I am not aware of any 9-2 Cr steel which has an actual or predicted rupture time of more than 1,000 hours. There is a need for a steel which will not rupture after 1,000 hours of service under these conditions.